1. Field of the Invention
This application claims priority to EPC application No. 01301116.8 filed Feb. 8, 2001, herein incorporated by reference.
2. Background of the Related Art
The present invention relates generally to lithographic projection apparatus and more particularly to lithographic projection apparatus including a controller to adjust a shape of a focal plane of the projection system.
In general, a lithographic projection apparatus in accordance with embodiments of the present invention includes a radiation system for supplying a projection beam of radiation; a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern; a substrate table for holding a substrate; and a projection system for projecting the patterned beam onto a target portion of the substrate, said projection system having a focal plane and comprising at least one adjustable element capable of changing the shape of the focal plane.
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to structure that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask according to the pattern on the mask. In the case of a mask the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning structure can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCF patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (eg comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously-scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this terns should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and U.S. Pat. No. 6,262,796 incorporated herein by reference.
To correctly image the mask pattern onto the substrate it is necessary to position the wafer accurately in the focal plane of the projection lens. The position of the focal plane can vary according to the position of the mask, illumination and imaging settings in the illumination and projection systems and due to, for example, temperature and/or pressure variations in the apparatus, during an exposure or series of exposures. To deal with these variations in focal plane position, it is known to measure the vertical position of the focal plane using a sensor such as a transmission image sensor (TIS) or a reflection image sensor (RIS) and then position the wafer surface in the focal plane. This can be done using so-called xe2x80x9con-the-flyxe2x80x9d leveling whereby a level sensor measures the vertical position of the wafer surface during the exposure and adjusts the height and/or tilt of the wafer table to optimize the imaging performance. Alternatively, so-called xe2x80x9coff-axisxe2x80x9d leveling can be used. In this method, a height map of (a part of) the wafer surface is taken, e.g. in a multi-stage apparatus, in advance of the exposure and height and tilt set points for the exposure, or series of exposures, to optimize the focus according to defined criteria, are calculated in advance. Methods and a system for such off-axis leveling are described in European Patent Application EP-A-1 037 117. In the off-axis method, it is proposed that the exact shape and position of the wafer surface be measured and the wafer height and tilt positions for the exposure can then be optimized to minimize defocus predicted relative to that measured wafer surface. Since the focal plane of the projection system will generally be flat and the wafer surface will generally not be flat, there will always be some residual defocus which cannot be compensated for by leveling procedures.
One aspect of embodiments of the present invention includes a system and a method for controlling a lithographic projection apparatus to further improve focus across the entire exposure area.
This and other aspects are achieved according to embodiments of the invention in a lithographic apparatus as specified in the opening paragraph, characterized by:
a controller, operative during an exposure for imaging the irradiated portion, to control said adjustable element to change the shape of said focal plane to more closely conform to the surface contour of said exposure area.
As discussed above, certain methods in which the focal plane is generally arranged to be as flat as possible and the substrate height and/or tilt are controlled to minimize defocus, inevitably leave some residual defocus as the wafer surface is generally not exactly flat. According to the present invention, rather than attempting to make the focal plane exactly flat, its shape is deliberately changed to make it conform more closely to the measured surface contour of the substrate in the exposure area to be exposed. Control of the wafer height and tilt is integrated with control of the shape of the focal plane. Then, low order (height and tilt) corrections can be effected by positioning the substrate and high order corrections can be effected by adjustments to the shape of the focal plane. Also, low order effects of high order adjustments to the shape of the focal plane can be compensated for in positioning of the substrate.
Embodiments of the present invention can therefore provide improved imaging by reducing defocus across the entire exposure area. This improves imaging quality on all exposure areas and also makes possible focusing on exposure areas having curved surfaces that would previously have exceeded defocus units.
Embodiments of the present invention can make use of all available manipulators in the projection system to adjust elements that affect the shape of the focal plane. Such manipulators are provided with suitable actuators, e.g. motors, piezoelectric actuators, solenoids, etc., to enable the a controller to adjust the elements to which the manipulators are connected. The adjustable elements may include elements specifically provided for the present invention or provided for other purposes such as correcting field curvature introduced by changes in magnification, or correcting astigmatisms in the lenses. The adjustable elements may have their position and/or orientation in any of the six degrees of freedom changed by the manipulators. Additionally, it is possible to adjust the shape of the element, e.g. where the element is a reflector provided with piezoelectric elements for adjusting its surface figure.
According to a further aspect of embodiments of the invention there is provided a device manufacturing method comprising the steps of:
providing a substrate that is at least partially covered by a layer of radiation-sensitive material;
providing a projection beam of radiation using a radiation system;
using patterning structure to endow the projection beam with a pattern in its cross-section; and
projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material using a projection system, said projection system having a focal plane and comprising at least one adjustable element capable of changing the shape of the focal plane;
characterized by the step of:
controlling said adjustable element during the step of imaging to change the shape of said focal plane to more closely conform to the surface contour of said exposure area.
Although specific reference maybe made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (eg. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, eg. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.